The present invention relates to an apparatus for sensing oxygen concentration wherein a solid electrolyte is employed for sensing oxygen concentration, and more particularly, to such an apparatus which is suitable for use in sensing the concentration of the oxygen contained in a combustion gas and thereby properly controlling the mixture ratio of air and fuel according to the running condition of an automobile.
Such apparatus for sensing oxygen concentration (hereafter referred to as an "O.sub.2 sensor") is used as a stoichiometric air fuel mixture ratio sensor in which an electromotive force is generated between a reference gas and an exhaust gas which are separated by a partition wall of a solid electrolyte made of, for example, zirconia, and which controls the mixture ratio of air and fuel supplied to an engine in the approximate ratio of 1:4.7.
A system was at one point proposed for improving the accuracy of an O.sub.2 sensor in which a cylindrical electrolyte which is closed at one end is kept heated by inserting a bar shaped heater therein, but this resulted in a relatively complicated apparatus requiring three terminals. More recently, there has been a proposal for a so-called lean O.sub.2 sensor as, for example, described in Japanese Patent Laid-Open No. 100746/1983. In this lean O.sub.2 sensor, a diffusion layer of 400 .mu.m to 500 .mu.m is formed on a cathode and a current of oxygen ions is forcefully pumped in an electrolyte between the cathode and the anode. This current is employed as a medium for sensing the degree of concentration of oxygen of the gas of concern relative to a reference gas. Such a lean O.sub.2 sensor requires at least four terminals. Two of these are required for a heater. The other two are necessary for supplying a current of oxygen ions since the cathode of the leads connecting to these terminals cannot be grounded because of the voltage drop which would occur if the cathode were grounded. In an O.sub.2 sensor of the above mentioned types including the lean O.sub.2 sensor, the electrode extending portion for a heater is Ni plated and the lead wires of nickel are secured by a silver solder. The secured portion by the silver solder, however, is oxidatively corroded in an oxidizing atmosphere, disadvantageously causing a natural separation to occur. It is known that, when a heater is included in the lean O.sub.2 sensor assembly and the sensor is mounted on the exhaust pipe of an automobile, the plug member of the lean O.sub.2 sensor comes to have a surface temperature as high as 500.degree. C. in the summer months when the automobile engine is idling after running at high speed. Therefore, if a sensor is used in such a high temperature oxidizing atmosphere, it is necessary to have a long heater so that the joint of the heater and its leads are separated from the distal end of the plug member to prevent the temperature of the heater lead joint from exceeding 200.degree. C. When a bar heater is made long, the heater includes a bend of, for example, 0.2 to 0.3 mm relative to its length of 50 mm. In this state, if the heater is inserted into the inner bore of a zirconia element, it is offset. This leads to deterioration in the temperature distribution of the element and lowering of the average temperature of the element which in turn results in a deterioration in the flow of oxygen ions and lowering of the output. Further, a long bar heater involves a correspondingly large bend in the assembly operation, disadvantageously increasing the possibility of breakage.